Many refrigeration systems can provide a variable supply of cooled air to cool multi-zone buildings. The amount of cooled air conveyed to each zone is often regulated by valves to meet each zone's cooling demand. Valves used for such a purpose are commonly referred to in the industry as VAV (variable air volume) valves.
A problem exists when one or just a few zones require heating while the rest of the zones still require cooling. Simply shutting off the cool supply air to these few zones is an unsatisfactory solution to the problem, because each zone requires at least some ventilation. Providing each zone with an additional supply air duct for heating is another possible solution, but one which is very expensive, especially when retrofitting an existing structure.